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Views: 222 Author: Carie Publish Time: 2025-04-26 Origin: Site
Content Menu
● Introduction to Sewage Treatment
● What is Secondary Sewage Treatment?
● Key Processes in Secondary Treatment
● Types of Secondary Treatment Systems
● Step-by-Step Breakdown of Secondary Treatment
>> 1. Influent from Primary Treatment
>> 3. Decomposition of Organic Matter
● Common Technologies in Secondary Treatment
>> Rotating Biological Contactors (RBCs)
>> Sequencing Batch Reactors (SBRs)
● Factors Affecting Secondary Treatment Efficiency
>> Temperature
>> pH
>> Hydraulic and Organic Loading
● Environmental and Regulatory Considerations
● FAQ
>> 1. What is the main goal of secondary sewage treatment?
>> 2. How does the activated sludge process work?
>> 3. What is the difference between aerobic and anaerobic secondary treatment?
>> 4. Why is secondary treatment important for environmental protection?
>> 5. What happens to the sludge produced in secondary treatment?
● Citation
Secondary sewage treatment is a critical stage in the wastewater treatment process that uses biological methods to remove dissolved and suspended organic matter from sewage. This article provides a comprehensive breakdown of secondary sewage treatment, including its processes, technologies, importance, and frequently asked questions. Visual aids and video resources are included to enhance understanding.
Sewage treatment is a multi-stage process designed to remove contaminants from wastewater, making it safe for release into the environment or for reuse. The main stages are:
- Preliminary Treatment: Removal of large debris and grit.
- Primary Treatment: Sedimentation to remove settleable solids.
- Secondary Treatment: Biological degradation of dissolved and suspended organic matter.
- Tertiary Treatment: Advanced purification for nutrient removal and disinfection.
Each stage progressively improves water quality, protecting ecosystems and human health.
Secondary sewage treatment, also known as biological wastewater treatment, focuses on the removal of biodegradable organic matter using microorganisms. This stage follows primary treatment, which removes larger solids and some organic matter through physical processes.
During secondary treatment, bacteria and other microorganisms consume organic pollutants, converting them into water, carbon dioxide, and new microbial cells. The effectiveness of this process is measured by the reduction in Biological Oxygen Demand (BOD) and Total Suspended Solids (TSS).
Secondary treatment is essential because primary treatment alone cannot sufficiently reduce organic pollutants to safe levels. Without secondary treatment, wastewater discharged into natural water bodies can cause oxygen depletion, harming aquatic life and posing public health risks.
Microorganisms, primarily bacteria, protozoa, and fungi, metabolize organic matter in sewage. This biological activity converts complex organic molecules into simpler substances such as carbon dioxide, water, and microbial biomass.
Aerobic treatment requires oxygen to sustain the metabolism of aerobic bacteria. Oxygen is supplied by mechanical aerators or diffusers, which introduce air into the wastewater. Aerobic bacteria are efficient at decomposing organic matter and are widely used in secondary treatment.
Anaerobic treatment occurs in the absence of oxygen, where anaerobic bacteria break down organic matter. This process produces biogas (mainly methane and carbon dioxide), which can be captured and used as an energy source. Anaerobic processes are more common in sludge digestion but are also applied in some secondary treatment systems.
After biological treatment, the mixture of treated water and microbial biomass (called mixed liquor) flows to a secondary clarifier. Here, gravity separates the solids (activated sludge) from the treated water. The clarified effluent is discharged or sent for further treatment, while the sludge is either recycled or removed.
Secondary treatment systems can be broadly classified into two categories based on how microorganisms are maintained:
| System Type | Description | Example Technologies |
|---|---|---|
| Fixed-Film (Attached Growth) | Microorganisms grow on media; sewage passes over it | Trickling filters, rotating biological contactors, constructed wetlands |
| Suspended-Growth | Microorganisms are suspended in the water | Activated sludge process |
In fixed-film systems, bacteria form a biofilm on surfaces such as rocks, plastic media, or other substrates. Wastewater passes over or through this biofilm, allowing microorganisms to degrade organic matter.
- Trickling Filters: Wastewater is sprayed over a bed of media where biofilms grow. Oxygen is naturally supplied by air flowing through the filter.
- Rotating Biological Contactors (RBCs): Discs partially submerged in wastewater rotate to expose biofilms alternately to air and sewage.
- Constructed Wetlands: Engineered ecosystems where plants and microbes work together to treat wastewater.
In suspended-growth systems, microorganisms are suspended in the wastewater itself.
- Activated Sludge Process: The most common suspended-growth system, where aeration tanks mix sewage with a microbial-rich sludge.
- Extended Aeration: A variation with longer aeration times to stabilize organic matter.
- Sequencing Batch Reactors (SBRs): Batch process reactors that alternate between aeration and settling phases.
Wastewater entering secondary treatment has already undergone preliminary and primary treatment, removing large solids and some organic matter.
Wastewater is introduced into an aeration tank or fixed-film reactor containing a large population of microorganisms. Oxygen is supplied in aerobic systems to support microbial metabolism.
Microorganisms consume organic pollutants, converting them into carbon dioxide, water, and new microbial cells. This reduces BOD and TSS significantly.
The mixed liquor flows to a clarifier, where biological solids settle out. The clear effluent is separated from the sludge.
A portion of the settled sludge (return activated sludge) is recycled to maintain microbial populations in the reactor. Excess sludge (waste activated sludge) is removed for further treatment, such as digestion or dewatering.
Depending on regulatory requirements, treated water may be disinfected using chlorine, ultraviolet light, or ozone before discharge or reuse.
The activated sludge process is the most widely used secondary treatment technology worldwide. It involves aerating wastewater in large tanks to support aerobic bacteria that metabolize organic matter.
Advantages:
- High efficiency in removing organic pollutants.
- Can be adapted for nutrient removal (nitrogen and phosphorus).
- Flexible operation for varying loads.
Challenges:
- Requires energy for aeration.
- Produces excess sludge that needs disposal.
- Sensitive to toxic substances.
Trickling filters consist of a bed of media (rocks, plastic) over which wastewater is distributed. Microorganisms form a biofilm on the media, degrading organic matter as wastewater trickles through.
Advantages:
- Simple design and operation.
- Low energy requirements.
- Resistant to shock loads.
Challenges:
- Larger land area required.
- Potential for clogging.
- Lower removal efficiency for some pollutants compared to activated sludge.
RBCs use rotating discs partially submerged in wastewater. The biofilm on the discs alternately contacts air and sewage, facilitating aerobic degradation.
Advantages:
- Compact footprint.
- Energy efficient.
- Easy to operate.
Challenges:
- Mechanical parts require maintenance.
- Limited capacity for high-strength wastewater.
SBRs treat wastewater in batches, cycling through fill, aeration, settling, and decanting phases in a single tank.
Advantages:
- Flexible operation.
- Good for small to medium-sized plants.
- Can achieve nutrient removal.
Challenges:
- Requires careful timing and control.
- Batch operation may limit throughput.
Microbial activity is temperature-dependent. Lower temperatures slow metabolism, reducing treatment efficiency. Most systems operate optimally between 10°C and 35°C.
Microorganisms thrive in neutral to slightly alkaline conditions (pH 6.5–8.5). Extreme pH values inhibit biological activity.
Adequate oxygen is critical for aerobic processes. Insufficient aeration leads to incomplete treatment and odor problems.
Heavy metals, disinfectants, and industrial chemicals can inhibit or kill microorganisms, disrupting treatment.
Overloading the system with excessive flow or organic matter can reduce treatment efficiency and cause sludge washout.
Regulatory agencies set limits on BOD, TSS, nutrients, and pathogens in treated wastewater to protect receiving waters. Secondary treatment typically aims to reduce BOD and TSS to below 30 mg/L.
Excess nitrogen and phosphorus cause eutrophication in water bodies. Some secondary treatment systems incorporate nutrient removal processes to reduce these pollutants.
Sludge generated during secondary treatment must be handled responsibly. Common methods include anaerobic digestion, composting, incineration, or land application following pathogen reduction.
Energy consumption, especially for aeration, is a major operational cost. Advances in energy-efficient aerators, renewable energy integration, and resource recovery (e.g., biogas) improve sustainability.
- Reduces Pollution: Removes up to 85% of organic matter, protecting water bodies and ecosystems.
- Prepares Water for Reuse or Discharge: Ensures effluent meets regulatory standards for BOD and TSS (typically <30 mg/L each).
- Protects Public Health: Reduces pathogens and harmful substances in wastewater.
- Facilitates Resource Recovery: Some processes generate biogas or recover nutrients.
- Supports Environmental Sustainability: Minimizes water pollution and conserves water resources.
Secondary sewage treatment is a vital biological process that removes dissolved and suspended organic matter from wastewater, significantly reducing environmental and health risks. By utilizing microorganisms in controlled aerobic or anaerobic environments, this stage achieves high removal rates of BOD and TSS, ensuring that effluent meets regulatory standards before discharge or further treatment. The process is essential for protecting water resources, supporting ecosystem health, and enabling the safe reuse of water.
Advancements in secondary treatment technologies continue to improve efficiency, energy use, and nutrient removal capabilities, making wastewater treatment more sustainable and effective. Understanding the breakdown and operation of secondary sewage treatment helps engineers, environmentalists, and the public appreciate the complexity and importance of managing wastewater responsibly.
The primary goal is to remove biodegradable organic matter and suspended solids from wastewater using biological processes, thereby reducing pollution and preparing water for safe discharge or reuse.
In the activated sludge process, wastewater is mixed with air and a microbial-rich sludge in an aeration tank. Microorganisms break down organic matter, and the mixture is then settled in a clarifier to separate treated water from sludge.
Aerobic treatment uses oxygen to support bacteria that decompose organic matter, while anaerobic treatment occurs without oxygen and produces biogas as a byproduct.
It removes up to 85% of organic pollutants, preventing water pollution, protecting aquatic life, and reducing health risks from pathogens and toxic substances.
Some sludge is recycled back to maintain microbial populations; the excess is removed and processed further, often through digestion, dewatering, or disposal.
[1] https://aquacycl.com/blog/secondary-treatment-of-wastewater-how-does-it-work/
[2] https://en.wikipedia.org/wiki/Secondary_treatment
[3] https://testbook.com/civil-engineering/secondary-treatment-of-wastewater
[4] https://www3.epa.gov/npdes/pubs/bastre.pdf
[5] https://www.doubtnut.com/qna/53725900
[6] https://www.khanacademy.org/science/ap-college-environmental-science/x0b0e430a38ebd23f:aquatic-and-terrestrial-pollution/x0b0e430a38ebd23f:waste-disposal-reduction-and-treatment/v/ap-es-sewage-treatment
[7] https://www.dsd.gov.hk/EN/Sewerage/Sewage_Treatment_Facilities/Type_of_Sewage_Treatment_Facilities/index.html
[8] https://www.geo.fu-berlin.de/en/v/iwrm/Implementation/technical_measures/Wastewater-treatment/Off-site-treatment/Sewage-Treatment-Plants/Secondary-Treatment/index.html
